A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
A lithographic apparatus of the type described above employs a plurality of actuators for positioning a part of the apparatus. Such actuators are applied for positioning a substrate table, a part of an irradiation system, a part of an illumination system or any other part of the lithographic apparatus. With this, a highly accurate relative positioning is desired, which is generally accomplished with the aid of a Lorentz actuator.
A Lorentz actuator includes an electrically conductive element, such as a coil, and a magnet assembly. The magnet assembly produces a magnetic field which interacts with a current flowing in the electrically conductive element to produce a Lorentz force between the electrically conductive element and the magnet assembly in a direction perpendicular to the direction of the current flow and the magnetic field at that point. Typically the magnet assembly includes at least one set of magnets on one or on either side of the electrically conductive element to produce an approximately uniform magnetic field around the electrical conductor. The magnet assembly includes a back iron for guiding a magnetic flux from one magnet to another. The back iron is formed from a material with high magnetic saturation and is located on the outward side of the magnets. The back iron is typically large to prevent saturation and it constitutes a substantial part of the mass of the actuator and is a source of loss of efficiency in the motor.
Currently known short stroke Lorentz actuators use the back iron not only to guide the magnetic flux from one magnet to the other, but also as a structural carrier. For this known back irons are constructed as substantially flat iron plates with magnets glued thereon.
However, iron as constructional carrier material is heavy and leads to heavy actuators, which leads to less acceleration and thus leads to a lower throughput. Furthermore, the dynamical performance and accuracy of the known actuators should be improved. Also the expansion of the iron when heated, because the motors heats up, is undesirable.